Robin Gibb, 61, was diagnosed with liver cancer several months ago, and has since cancelled several public appearances.
The singer spent five hours in hospital on Tuesday before being allowed back home, where he was joined by his mother Barbara, 91, and his older brother Barry.
Barry Gibb has flown in from the US to be with his sibling.
It has been no secret that Robin Gibb has become frail over the last few months.
The singer was discharged after four days, but said to be in severe pain.
Robin Gibb lives in an £8 million ($12 million) 12th Century home in Thame, Oxon,with his wife Dwina.
“Robin is not good and there is a lot of concern for him,” a friend has said, according to the Sunday Mirror.
“You can use your wealth to call in the best experts but sometimes no amount of fame, prestige and money can change things when it comes to cancer.
“But Robin is a strong character, he is a fighter and has been encouraged by all the online messages from his fans.
“Dwina is doing everything possible and hasn’t left his side.”
His wife is reportedly trying to persuade Robin Gibb to visit a Native American medicine man.
“This incredible Indian tribe introduced Robin and I to something called Spider Medicine that apparently contains properties that can help you get well from certain untreatable illnesses,” said the druid priestess.
“There will be difficult times ahead but Robin will never give up and his loving family will make sure he has everything he needs. There is frustration because Robin has always looked after himself. He doesn’t drink, eats well and exercises daily,” the friend added.
Robin Gibb described Maurice’s death at 53 as “something I haven’t accepted”, adding:
“I just imagine he is out there somewhere and I will bump into him one day.”
The Bee Gees disbanded after Maurice’s death, having sold 200 million records worldwide with hits such as Night Fever and Stayin’ Alive.
Robin Gibb’s younger brother Andy died in 1988 aged 30 after struggling with a cocaine addiction.
“It is incredibly sad for Barbara. First she lost her son Andy, then Maurice and now she has to cope with seeing Robin like this,” said a friend, according to the Sunday Mirror.
“It is a lot for a mother, particular of her age, to deal with. Barry has also been there for his brother and a lot of friends have been rallying round.”
Robin Gibb cancelled a US tour in April due to extreme stomach pain.
The star had part of his intestine removed in an emergency procedure last year, after suffering blockage similar to that which killed his brother Maurice.
Robin Gibb was quick to squash claims last month that his condition was worsening, saying: “I’m feeling great, absolutely great. I’m working on the Titanic Requiem at the moment and it’s going really well.“
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